Device could convert waste heat into electricity

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An MIT scientist and a colleague have invented a semiconductor technology that could allow efficient, affordable production of electricity from a variety of energy sources--including waste heat--without a turbine or similar generator. The researchers presented the work at a Nov. 27 poster session during the Materials Research Society's fall meeting in Boston.

Many researchers have worked to convert heat to electricity directly without using the moving parts of a generator. Among other advantages, such a device would be virtually silent, vibration-free and low in maintenance costs. Until now, however, the efficiency of such devices has been a problem. The amount of electricity they produce from a given amount of energy has been low.

The new device is twice as efficient as its closest commercial competitor. "That such good results were obtained in the first generation of the new device technology ... indicates that the general approach has great promise for improved performance in more mature implementations," wrote Associate Professor Peter L. Hagelstein of electrical engineering and computer science and Dr. Yan Kucherov of ENECO, Inc., in the paper accompanying their poster.

The new technology could have major implications for the recovery of waste heat from power plants and automobiles. For example, the heat lost through engine exhausts might be captured by the technology and converted into electricity to augment or replace a vehicle's electrical and air conditioning systems. It could also be important in the primary generation of electrical power.

The technology is based on thermionics, which originated nearly a century ago with the basic vacuum tube, which consisted of two parallel conductive plates (cathode and anode) separated by a vacuum gap. In this high-temperature tube, electrons boiled off the cathode, traversed the gap and then were absorbed into the colder anode. The conversion of heat to electricity "occurs as the electrons transport 'uphill' against an electric field in the gap region," said Hagelstein, who is also affiliated with Research Laboratory of Electronics.

These early "vacuum gap" designs had prohibitive manufacturing costs and high operating temperatures--above 1,000 degrees C (about 2,000 degrees F)--which have limited the technology to nuclear-powered converters in space probes, satellites and special military systems.

The new technology essentially replaces the traditional vacuum gap with a multilayer semiconductor structure. Hagelstein credits Professor Gerald D. Mahan of the University of Tennessee with first suggesting such a solid-state implementation of vacuum thermionics. Hagelstein and Kucherov demonstrated two basic enabling physical mechanisms that allow this technology to be implemented practically.

Louis D. Smullin, professor emeritus of electrical engineering, said of the new work: "Thermocouples and thermopiles have been with us for over a century. I believe that these new devices represent the first big step in performance of these devices. In the 50s there was much hope that direct conversion of heat to electricity would open up a new era, but it was not to be. With these new devices, maybe these dreams will come true."

By careful selection of materials, ENECO scientists are creating highly efficient, solid-state conversion devices, called "thermal diodes," that will operate from 200 to 450 degrees C--typical temperatures for waste heat and for concentrated solar radiation.

An added plus: the technology is environmentally friendly. "Solid-state thermal-to-electric energy conversion converts energy due to how electrons transport in the conductor, a process that generates no pollution," Hagelstein said. He noted, however, that some of the materials used in the present generation of devices are toxic, which will affect the eventual disposal of the devices.

The work has been sponsored by ENECO with additional support from the Defense Advanced Research Projects Agency. Technical development is now focused on optimizing the types of materials used in buliding the thermal diodes.

Hagelstein is a technical consultant for ENECO, which is developing the technology and has applied for patents in the United States and Europe. At least one patent has been issued.

A version of this article appeared in MIT Tech Talk on December 5, 2001.